CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in immune defense against infectious agents, tumors and transplants. Class I-restricted CD8+ T cells have been implicated in the recognition and destruction of such clinically important targets as HIV-1 (1–3), Influenza A (4), malaria (5), cytomegalovirus infected cells (6), Epstein-Barr virus (7), and human melanoma cells (8–9). Therefore, establishing methods for inducing and expanding populations of antigen-specific CD8+ CTLs remains an important objective in the development of therapeutic treatments against infectious disease and cancer.
CD8+ CTLs are activated by antigens which have been processed and presented on major histocompatibility (MHC) class I molecules on the surface of specialized antigen presenting cells. A number of antigen presenting cells have been identified which activate T cells including macrophages/monocytes, B cells and bone marrow derived dendritic cells. Of these, dendritic cells are recognized as playing a pivotal role in the initiation of CD8+ CTL responses (10).
An important feature of dendritic cells is their ability to efficiently process and present antigens on MHC class I and/or class II molecules. Depending on the antigen processing pathway, dendritic cells are capable of activating distinct populations of CTLs. In the case of influenza virus, for example, it is known that the class I pathway for inducing CD8+ CTLs requires adequate delivery of infectious viral antigen into the cytoplasm, whereas the purely endocytic pathway delivers noninfectious virions for presentation only to CD4+ T helper cells (U.S. Ser. No. 08/282,966). Thus, although dendritic cells efficiently activate class I-restricted CTLs, access to the MHC class I pathway for induction of CD8+ T cells normally requires endogenous synthesis of antigen. Accordingly, it is important to identify antigen delivery systems which efficiently mediate access of exogenous antigen to the MHC class I-restricted antigen presentation pathway in order to generate antigen-specific CD8+ T cell responses.
Recently, a number of approaches have been reported for delivery of exogenous antigen to the MHC I processing pathway of dendritic cells. These methods include coupling antigens to potent adjuvants (11–15), osmotic lysis of pinosomes after pinocytic uptake of soluble antigen (16), or insertion of antigen in pH-sensitive liposomes (17). However, these prior approaches still pose a number of limitations on the development of effective therapeutic treatments. For example, Nair et al. report that dendritic cells do not efficiently internalize antigen-containing liposomes in vivo (18). Further, osmotic lysis of dendritic cell pinocytic vesicles may be difficult to perform in vivo, and may result in inefficient antigen delivery to the MHC class I processing compartment. Also, the use of powerful adjuvants may be undesirable for some clinical applications.
Pulsing dendritic cells directly with exogenous antigen using whole cells in viable or irradiated forms, membrane preparations, or antigens purified from natural sources or expressed as recombinant products has also been previously reported (WO 94/02156). These prior methods, however, do not recognize forms of cell death or the processing pathways antigens from dead or dying cells access in the dendritic cell system.
Rubartelli et al. report that dendritic cells, unlike macrophages, fail to take up opsonized particles or necrotic cells in vitro, but can efficiently engulf cells undergoing apoptotic programmed cell death (19). The mechanism of internalization of apoptotic cells by dendritic cells, however, is different than in macrophages, indicating that results in the macrophage system are not necessarily predictive of dendritic cell responses. In addition, Rubartelli et al. does not show presentation of engulfed material and therefore speculates as to the fate of such material.